Cardiac rhythm management devices are implantable battery-powered devices that provide electrical stimulation to selected chambers of the heart in order to treat disorders of cardiac rhythm. A pacemaker, for example, is a cardiac rhythm management device that paces the heart with timed pacing pulses. The most common condition for which pacemakers have been used is in the treatment of bradycardia, where the ventricular rate is too slow. If functioning properly, the pacemaker makes up for the heart's inability to pace itself at an appropriate rhythm in order to meet metabolic demand by enforcing a minimum heart rate and/or artificially restoring AV conduction. Implantable devices may also be configured to treat tachyarrhythmias such as tachycardia and fibrillation with electrical stimulation. An implantable cardioverter/defibrillator (ICD) provides this kind of therapy by delivering a shock pulse to the heart when the device detects fibrillation. Another type of electrical therapy for tachyarrhythmias is anti-tachycardia pacing (ATP). In ATP, the heart is competitively paced with one or more pacing pulses in an effort to interrupt the reentrant circuit causing a tachycardia. ATP can be applied to either the ventricles or the atria. Modern ICD's typically are also pacemakers with ATP capability configured so that ATP therapy is delivered to the heart when a tachycardia is detected, while a shock pulse is delivered when fibrillation occurs.
The delivery of pacing pulses to heart, whether to treat bradycardia or tachycardia, is a transfer of energy that can have deleterious physiological effects if sustained over a long enough period and at a high enough rate. In order to prevent this possibility, it would be desirable for a pacemaker to be configured to monitor the rate at which pacing energy is transferred as well as the cumulative amount of the energy as the pacemaker delivers pacing therapy in different modes.